1. Field of the Disclosure
The present invention relates generally to power supplies and, more specifically, the present invention relates to alternating current (AC) to direct current (DC) power supplies.
2. Background Information
A current having a frequency that is a multiple of the fundamental frequency of an AC power line is a harmonic current. Harmonic currents are a burden to the power generation and distribution system. Hence, regulatory agencies impose restrictions on the harmonic current that power supplies draw from the power line.
It is well known that switching power supplies can be controlled so that the input current is directly proportional to the input voltage, making the power supply appear as a resistor to the power source. These techniques have been successful in reducing the harmonic current at the input of switching power supplies that use many different topologies, such as for example the boost, buck-boost, flyback, and single ended primary inductor converter (“SEPIC”) topologies that are known in the art. In each of these topologies, the technique of making the power supply appear as a resistor to the power source is sufficient to allow an AC-to-DC power supply to meet the requirement of low harmonic current at the input.
The same technique that makes a power supply appear as a resistor to a power source, however, does not meet the requirement for low harmonic current when it is applied to a buck switching converter topology. Whereas other converter topologies can take current from the input during an entire cycle of the AC input voltage, the buck converter takes current from the input only when the input voltage is greater than the output voltage. Therefore, the power supply appears as a resistor only during a portion of the cycle of the AC input voltage, and appears as an open circuit otherwise. As a result, the AC input has higher than acceptable harmonic current.